Highly-phosphorescent tungsten(0) carbonyl pyridyl-imidazole complexes as photosensitisers

Photoluminescence data are reported for two W(CO)₄L complexes (L = 2-(1H-imidazol-2-yl)pyridine and 2-(2'-pyridyl)benzimidazole) in room-temperature solutions. The bidentate ligands consist of a pyridine and an imidazole moiety connected by a C-C bond. The complexes have been found to exhibit enhanced phosphorescence from the metal-to-ligand charge transfer (MLCT) excited state with quantum yields in the order of 10^-3, almost two orders of magnitude higher than those reported for W(CO)₄(diimine) complexes. One of the complexes W(CO)₄(2-(2'-pyridyl)benzimidazole) can serve as an efficient visible-light photosensitiser for the isomerisation of aromatic alkenes with efficiency comparable to and even exceeding that of the well-studied ruthenium tris-bipyridine complex, Ru(bpy)Cl₂.